Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an I.sup.2 T. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen by another electrostatic field to produce a visible image.
A power supply for the I.sup.2 provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate(s). Conventional night vision devices (i.e., since the 1970's and to the present day) provide automatic brightness control (ABC), and bright source protection (BSP). BCP maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in infrared and the near-infrared portion of the spectrum) of the scene being viewed. BSP prevents the I.sup.2 T from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view.
BSP and sometimes even ABC can be implemented by reducing the voltage on the photocathode as the intensity of the scene being viewed increases. Changes in this intensity are typically reflected by changes in the overall current flowing through the photocathode.
As a practical matter, however, the voltage on the photocathode cannot be reduced below a threshold level called the charge voltage for the tube. The charge voltage is the minimum level of voltage which is necessary for the photocathode to liberate electrons of sufficient energy to penetrate the ion barrier at the front face of the microchannel plate. If the applied voltage is less than the charge voltage, the photocathode will not function at all.
The circuitry which reduces the voltage applied to the photocathode in response to high intensity scene levels, therefore, must insure that the applied voltage does not drop below the charge voltage. In the prior art, this has typically been done by a clamping circuit which clamps the voltage applied to the photocathode to no less than a pre-determined minimum amount.
This prior art clamping circuit, however, provides far less that an ideal solution. The problem lies in the fact that the charge voltage typically varies substantially for photocathodes of the same type. To insure that no photocathode is disabled by the voltage-reducing circuitry, the clamping voltage must therefore be set at a level which is higher than the highest value of anticipated charge voltage in the entire set of photocathodes.
Setting the clamping voltage at this high level results in many photocathodes receiving a minimum voltage level far above the level which would be ideal for these photocathodes. This forces these photocathodes to operate at an unduly high current level under very bright conditions, degrading the resolution and reliability of the photocathodes. The problem is particularly acute for today's performance tubes which are much more photo-sensitive.